In accordance with the European standard EN 81-20 “Safety rules for the construction and installation of lifts”, an elevator intended for the transport of loads such as goods and persons must have a specified rated load. Item 14.2.5 of the standard requires that the elevator must be provided with a device which prevents normal starting, including relevelling, when there is an overload in the car. In accordance with the definition, exceeding the rated load by 10%, however, at least by 75 kg, is deemed to be an overload. The above-mentioned standard alone requires that it is necessary to determine the load of the car.
The load of the car has conventionally been determined for example by means of a weighing sensor attached to the floor of the car or to the elevator ropes.
The applicant's Finnish patent application 20080535 discloses a control method for the elevator motor, by which control method the elevator can be driven without previously known load information. In the method, the mechanical brake of the elevator is opened, and by controlling the controllable electronic switches of the power supply equipment, the motor current is adjusted in order to keep the car in place in the elevator shaft. The load of the elevator is deduced from the motor current or from the power/moment reference. If the load of the car exceeds the specified rated load of the car, an overload situation of the car is deduced.
International patent application publication WO 2009/087266 A1 discloses a method for the determination of the load of a car. The determination of the load is adapted in conjunction with the movement control of the elevator system. When the machinery brakes of the elevator motor are opened, load determination begins to determine the position deviation of the elevator motor. The deviation is determined by comparing the position of the rotor of the elevator motor to the initial position that the rotor had when the determination began. A speed reference of the elevator motor during the determination of load is formed on the basis of this comparison, and the speed reference is exported to the speed controller.
Japanese patent H03293277 discloses an elevator where an overload is detected by means of a device which disconnects the operation of the elevator by means of a relay and activates alarm until the load is reduced.
Japanese patent 2010143692 discloses an elevator with a calculation of the number of elevator passengers and with a calculation of the number limit based on the detection of the overload of the motor controller.
United States patent application publication US 2010/0133046 A1 discloses an elevator control system which in an overload situation prevents the doors from closing until the load of the elevator is reduced to the required level.